1. Field of the Invention
The present invention relates to a coplanar waveguide used for interconnecting integrated circuit elements operating in the millimeter-wave frequency band or connecting such circuit elements to package connectors, and to a coplanar waveguide fabrication method.
2. Description of the Related Art
Microstrip lines and coplanar waveguides are currently used as propagation lines for microwave and millimeter-wave signals. A microstrip line has a ground conductor on the underside of the substrate. Accordingly, grounding on the upper surface of the substrate requires the formation of a via hole through the substrate. A coplanar waveguide has ground conductors formed on the upper side of the substrate, so all grounding is on the upper surface of the substrate and there is no need for via holes. Integrated circuits with coplanar waveguides can therefore be fabricated more easily and at a lower cost than integrated circuits with microstrip lines.
In a monolithic microwave integrated circuit (MMIC) with active elements such as mixers and amplifiers and passive elements such as filters and capacitors, coplanar waveguides are used to interconnect the active and passive elements. Such coplanar waveguides generally comprise metal wiring patterns, and the substrate is generally a compound semiconductor substrate such as a gallium arsenide (GaAs) or indium phosphide (InP) substrate. One advantage of a compound semiconductor substrate is that its high electron mobility permits the formation of active devices, such as metal-semiconductor field-effect transistors (MESFETs) and high electron mobility transistors (HEMTs), that can operate in the hundred-gigahertz (100-GHz) band. Another reason is that it is easy to fabricate compound semiconductor substrates having a resistivity as high as about ten million ohm-centimeters (107 Ω·cm).
Monocrystalline compound semiconductor substrates are, however, more expensive than monocrystalline silicon (Si) semiconductor substrates. Moreover, commercially available monocrystalline compound semiconductor wafers are generally only three to four inches in diameter, whereas ten-inch monocrystalline silicon semiconductor wafers are readily available. Because of the high cost and small size of monocrystalline compound semiconductor wafers, MMICs formed on monocrystalline compound semiconductor substrates are expensive.
In Japanese Patent Application Publication No. 2000-068714, Matsumoto has described the formation of coplanar waveguides in which the signal line and ground conductors are both disposed on an insulating film such as a silicon oxide film, a silicon nitride film, or a polyimide film at least ten micrometers (10 μm) thick, formed on a monocrystalline silicon semiconductor substrate with a resistivity of one thousand to ten thousand ohm-centimeters (1 kΩ·cm to 10 kΩ·cm). The insulating film reduces leakage of electromagnetic wave energy into the substrate, so that an MMIC with coplanar waveguides of this type can operate at frequencies in excess of 10 GHz despite the use of a silicon substrate.
Experiments performed by the inventor have shown that if the thickness of the insulating film is reduced to about 0.2 μm to 2 μm, the attenuation constant in the 1-GHz to 30-GHz band degrades by at least one decibel per millimeter (1 dB/mm). The reason is that a low-resistivity layer with a resistivity of about 0.01 Ω·cm forms at the interface between the silicon oxide or silicon nitride insulating film and the high-resistivity monocrystalline silicon substrate. To nullify the effect of this low-resistance layer, the insulating film must be at least about 10 μm thick.
Forming a silicon oxide or silicon nitride film at least 10 μm thick by plasma chemical vapor deposition (CVD), takes at least four to twelve hours, however, which is impractical for commercial fabrication.
For practical fabrication, accordingly, the coplanar waveguides need to be formed directly on the high-resistivity silicon wafer substrate, without an intervening insulating film, but further experiments performed by the inventor have shown that at frequencies of 60 GHz to 80 GHz, the attenuation constant of such coplanar waveguides exceeds 1 dB/mm, and the attenuation constant shows significant variations over the wafer surface.